Hybrid and electric vehicles (HEVs) typically include an electric traction drive system that includes an alternating current (AC) electric motor which is driven by a power converter with a direct current (DC) power source, such as a storage battery. Motor windings of the AC electric motor can be coupled to inverter sub-modules of a power inverter module (PIM). Each inverter sub-module includes a pair of switches that switch in a complementary manner to perform a rapid switching function to convert the DC power to AC power. This AC power drives the AC electric motor, which in turn drives a shaft of HEV's drivetrain. Traditional HEVs implement multiple three-phase pulse width modulated (PWM) inverter modules and multiple three-phase AC machines (e.g., AC motors) each being driven by a corresponding one of the three-phase PWM inverter modules that it is coupled to.
Vector Control
Many modern high performance AC motor drives use the principle of field oriented control (FOC) or “vector” control to control operation of the AC electric motor. In particular, vector control is often used in variable frequency drives to control the torque applied to the shaft (and thus finally the speed) of a three-phase AC electric motor by controlling the current fed to the three-phase AC electric motor. In short, stator phase currents are measured and converted into a corresponding complex space vector. This current vector is then transformed to a coordinate system rotating with the rotor of the three-phase AC electric motor. This technique requires knowledge of the rotor's angular position (i.e., the mechanical rotational angular position of rotor relative to the “stator” or motor windings).
Sensor-Based Control
Many vector controlled motor drive systems employ a rotor speed or position sensor, and a motor winding temperature sensor attached to the motor to control and protect the motor. For example, the rotor's angular position can be computed based on actual measured quantities using some type of speed or position sensor for control feedback measurement. For instance, to determine the angular position of the rotor, its angular speed can be measured with a speed sensor, and the angular position can then be obtained by integrating the speed measurements. Other systems may use a resolver and resolver-to-digital converter circuit which provides absolute position information directly. A speed or position sensor is an expensive component. These sensors typically increase the cost and reliability of the motor drive system due to the fact that extra sensor components and additional wiring are required. Moreover, the circuitry required to process its signals can also be expensive. These sensors can also have reliability and maintenance issues. The presence of the speed/position sensor in the system adds cost, size, and weight, and reduces reliability as well. As such, it would be desirable to eliminate these sensors. It would be desirable to eliminate this speed/position sensor and replace the measured quantities by computed estimates. It would also be desirable to eliminate mechanical interface hardware, reduce cost and weight, and improve the reliability of an electric traction drive system.
Sensorless Control
To eliminate the rotor position sensor, the rotor's angular position can also be estimated without actually using a sensor to measure actual speed/position quantities. Numerous methods for estimating the rotor's angular position have been developed The methods can generally be classified as those that work better at low motor operating speeds and those that work better at high motor operating speeds.
In this regard, numerous methods for estimating the rotor's angular position have been developed that are suitable for zero/low speed operation. Examples of sensorless control techniques that work well at low motor operating speeds (or zero speed) have been proposed, in United States Patent Application Publication Number US 2004/0070362 entitled “POSITION SENSORLESS CONTROL ALGORITHM FOR AC MACHINE,” filed Oct. 10, 2002, which is incorporated by reference herein in its entirety. The sensorless control technique described in US 2004/0070362 employs a high-frequency signal-injection method. In the high-frequency signal-injection method, a pulse width modulated (PWM) inverter injects a balanced high-frequency test signal based on the estimated rotor position, such as a voltage (or current) signal, on a stator winding of the motor. The resultant effect of the balanced high frequency test signal on stator current (or voltage) can be measured at the same position. The effect of the balanced high frequency test signal injection can be observed in a measured stator current which contains the position error information between the real and the estimated rotor position. This effect is due to the spatial modulation of the magnetic saliency as the rotor rotates. The current signal is detected at the corresponding frequency, and demodulated to reconstruct the rotor position and speed by using the low-pass filters and position/speed estimator.
At high motor operating speeds, where the back electromagnetic-motive-force (EMF) voltage is sufficiently high, a sensorless control technique that employs a math-based flux observer can be used. One such example is described in United States Patent Application Publication Number US 2009/0140676-A1 entitled “METHOD AND SYSTEM FOR SENSORLESS CONTROL OF AN ELECTRIC MOTOR,” filed Nov. 29, 2007, which is incorporated by reference herein in its entirety.
One approach that can be used to cover the entire range of motor operating speeds (i.e., rotor angular velocities) is to selectively enable/disable two different sensorless control techniques depending on the motor speed. For example, at high motor operating speeds, a sensorless control technique that employs a math-based flux observer, such as that shown in United States Patent Application Publication Number US 2009/0140676-A1 can be enabled. At low motor operating speeds (or zero speed), a sensorless control technique that employs a signal-injection method, such as that described in United States Patent Application Publication Number US 2004/0070362, can be enabled.
Conventional and hybrid electric vehicles have an oil pump that generates oil pressure needed to drive numerous actuators, such as clutches, pedals and many other hydraulic loads. It would be desirable to drive the oil pump with an electric permanent magnet (PM) motor that is common in such vehicles. It would be highly desirable to do so using a vector controlled motor drive system that implements sensorless control techniques described above in the background section to thereby eliminate the need for expensive position/speed sensors or motor temperature sensors. Although it would be desirable to use a PM motor and vector controlled motor drive system that uses sensorless control techniques to drive an oil pump, there are a number of considerations that make this quite challenging.
In comparison to other types of vector controlled motor drive systems (e.g., where the electric motor drives a vehicle drive shaft as opposed to where the electric motor drives an oil pump transmission), the oil pump transmission typically has a very small inertia, which makes it a highly dynamic mechanical system. In other words, the mechanical time constant (τ) is very small. The time constant determines the bandwidth of the system. Sensorless control techniques require high estimation bandwidth to track such fast mechanical movement.
Because viscosity of oil changes significantly with respect to the temperature, the load characteristics of the oil pump tend to vary significantly depending on temperature of the oil being pumped. For example, when the oil being pumped gets cold, its viscosity increases (e.g., becomes slushy) and the load dynamics of the motor become relatively slow in comparison to when the oil is at a higher temperature. This makes the rotor of the motor rotate more slowly with the same torque used at higher temperature. At high temperatures, however, the oil becomes thin, and the load dynamics become relatively fast. Thus, when the oil is cold, a much higher static torque will be required for the motor than in comparison to a higher temperature case, and in such a case, high bandwidth of a position and speed estimator in the United States Patent Application Publication Number US 2004/0070362 and US 2009/0140676-A1 will just amplify the noise even though the actual system moves slowly. By contrast, when the oil is hot, the oil pump driven by the electric motor has to be robust to this load variations, which becomes an issue for sensorless control.
Moreover, most oil pump control systems can use a speed controller. The speed controller is driven by a speed command, and the higher level controller controls pressure and flow of the oil being pumped in response to the speed command. However, when the speed command changes too abruptly, it will create the sudden back pressure which makes the motor stop momentarily, or may even spin the motor backward for a short time. When the actual motor speed changes too fast, this can induce control instability in sensorless control techniques that have finite bandwidth to estimate the rotor's position and angular velocity.
Dynamic performance of the sensorless control techniques described in the published applications mentioned above can be somewhat limited. For example, at higher bandwidths and motor speeds, a rotor position and speed estimator can track/estimate the position and speed of the rotor relatively well, but can exhibit control instability due to the noise. At lower bandwidths and motor speeds, when the motor delivers high torque, control stability is generally better since the rotor position and speed estimator can filter out unwanted noise (e.g., induced by the analog circuits). Nevertheless, the system may exhibit control instability when dynamic variation of the load changes too rapidly.
Accordingly, it would be desirable to enhance or improve conventional sensorless control techniques, and provide improved methods, systems and apparatus for sensorless control that can be used in a vector controlled motor drive system that includes an electric motor used to drive an auxiliary oil pump. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.